-We can figure out what a star is composed of by measuring the frequency of the radiation that it emits.​

-We know about fusion through experiments and, once we figure out what a star is composed of, we can apply fusion to the stars. In stars, hydrogen nuclei ram into each other and fuse to form helium nuclei. In the process of doing so, they release heat and light which in turn releases pressure and pushes the star outwards. Once there is no longer enough hydrogen to support the mass of the star, it collapses in on itself and, depending on how large the star was, could either become a white dwarf, neutron star, or black hole.​

The standard model of particle physics includes three fundamental forces, one of which is the weak nuclear force. The weak nuclear force deals with all forms of radioactive decay and it is responsible for hydrogen fusion in the stars.

Staff: Mentor

Thank you for the reply. Stars with huge mass when they collapse form black holes. But what about hydrogen and neutron star? How they are formed?

-- Shounak

Neutron star formation has to do with the size of the star that underwent the supernova explosion. There is a limit known as the Chandrasekhar limit that dictates the fate of a dying star. This limit is 1.44 solar masses. Any star whose mass is equal to or less than this will collapse to form a white dwarf. The Pauli exclusion principle states that no two identical fermions (electrons in this case) can be at the same place at the same time. When stars collapse, the atoms inside are pushed closer and closer together. In order to avoid breaking the Pauli Exclusion Principle, atoms add more energy levels to accomodate the "invasive" electrons and, in the process of doing so, release pressure. [Electron (Quantum in a more general sense) Degeneracy Pressure.]

If the star is greater than 1.44 solar masses, Electron Degeneracy Pressure will not be sufficient enough to overcome the gravitational collapse. At this point, electron capture occurs. During electron capture, the protons in the nuclei absorb the electrons and are converted into neutrons (and by doing so, release Neutron Degeneracy Pressure.) This process converts the majority of protons into neutrons, hence the name "neutron star." Neutron stars are incredibly small and incredibly dense. The neutron star is supported from further collapse by neutron degeneracy pressure as a result of the pauli exclusion principle.

For reference (more technical:)

Electron capture is a form of Beta Decay. The equation for electron capture is:

p + e- = n + ve

In word form, a proton absorbs an electron and converts into a neutron while simultaneously emitting an electron neutrino VIA Weak Nuclear Interaction.

The initial size of a star formed from a collapsing gas cloud is limited by the Jeans mass. This is a diet plan for newly forming stars. If the gas cloud is too massive [the guess on limits in the current enviornment of the universe is around 200 solar], it will fragment. In the early universe this mass may have been much larger, possibly several hundred or more solar masses for pop III stars, when metallicity of the gas cloud was negligible.